In the early years of pacemaker therapy, simple systems stimulating in the ventricle were used. This was due to the technical possibilities existing in the early sixties. Nevertheless, one soon recognized the value of an atrioventricular synchronization from a hemodynamic point of view. As early as 1963, Nathan described the application of an atrially triggered ventricle pacing system in the article "An implantable synchronized pacemaker for the long-term correction of complete heart block"0 in Circulation 23, 1963, pp. 682ff. This article made clear the advantages of rate control with simultaneous atrioventricular synchronization by detecting the atrial potential. Such pacemakers are advantageous in particular for patients with a complete AV block, i.e. if the stimulus conduction system in the heart is interrupted, that maintains the normal rhythm if the sinus node function still exists at the same time. Problems in reliably detecting the atrial signal led in past years to the development of an atrial screw electrode which is "screwed" into the atrial wall with a corkscrew-like spiral or the like. As of the mid seventies it became possible to detect the atrial signals with so-called VAT pacemakers and use them to trigger cardiac stimulation in clinical application.
However, the theoretical advantage of applying such atrially triggered VAT systems, which are known today as DDD systems or dual chamber systems, is opposed by considerable problems in practice. Firstly, the problem of firmly anchoring the electrodes in the atrium has not been satisfactorily solved, whether by screw electrodes or by inserting an electrode into the auricular appendix. The second problem relates to the instability of the atrial rhythm. A survey of this can be found in the book by E. Alt, "Schrittmachertherapie des Herzens," perimed Verlag, Erlangen (1988), pp 94 to 99 . In many patients showing a disturbance of stimulus conductance, there are also disturbances in the stimulus formation. This means that, alongside an occasionally regular sinus rhythm, there may be fast atrial arrhythmias in the sense of atrial fibrillation or an atrial flutter, but that sinus node dysfunction may also express itself as an excessively slow sinus node function. This is referred to as a "brady-tachy syndrome" or , if the AV conduction is additionally disturbed, as binodal disease. In the case of fast atrial arrhythmias the ventricle is paced inadequately in the rhythm of the atrial arrhythmia; in the case of sinus node damage and a deficient rise in the sinus rate the pacing rate for the ventricle is too slow. Technical restrictions with respect to a reliable detection of the atrial signal and due to the limited expressive power of the atrial signal lead, for a considerable number of patients, to restrictions in the dual chamber or DDD systems used at present.
To avoid the problems with respect to an unstable anchoring of the atrial electrode, it has been proposed to provide a floating pair of electrodes in the atrium within one pacing lead. This concept was already presented in 1979 by Antonioli in the book "Cardiac pacing, PACE Symposium, Montreal," edited by C. Meere, in the article "A simple P-sensing ventricle stimulating lead driving a VAT generator." U.S. Pat. No. 4,313,442 (Knudson) describes a corresponding pacemaker which integrates the atrial pulse signals and adjusts a pacing rate in accordance with the integrated atrial signal. This method does not allow for direct synchronization between the atrial beat and the ventricular beat, but for an adaptation of the ventricular beat rate to the mean atrial rate.
The concept of detecting the atrial signals indirectly with a multipolar electrode corresponds to the technique, that has been practiced for many years in electrophysiology with transitory electrodes, of detecting the atrial signal with bipolar, quadripolar or six-pole floating electrodes that pass through the atrium by lying in the blood stream and are not secured by direct contact to the atrial wall. Two electrode points, i.e. a pair of electrodes in the ventricle and one or two pairs of electrodes in the atrium, are generally used within a common electrode body. More recent developments in electrode technology also allow for the application for thin multipolar electrodes whose diameter is only about 1.6 millimeters.
In the past it was not easy for the evaluating electronics of cardiac pacemakers, however, to detect these indirectly detected signals of the atrium with a floating electrode position. But now that microprocessor technology has made progress, it is possible to derive a control signal for ventricular pacing triggered in synchronism with the atrium within a DDD system by means of suitable input filters and input amplifiers as well as appropriate processing of the signal perceived indirectly in the atrium from floating electrode points within the blood stream.
When the problem of detecting the atrial signal with one electrode not in direct contact with the atrial wall is solved, however, one still has the problem of the instability of the cardiogenic atrial signal, which, as explained above, may be both too fast and too slow. Since the ventricular rate is adapted to this atrial signal in a classic DDD pacemaker, this may result in an inadequately fast or slow ventricular rate.
To obtain, independently of the atrial rate, a correct pacing rate in the ventricle that is adapted to the patient's exercise, various concepts for rate adaptive pacemakers have been proposed in the past.
Krasner describes in U.S. Pat. No. 3,593,718 a pacemaker in which the external thoracic impedance is measured, and the breathing rate detected therefrom and used for controlling the pacing rate.
Nappholz describes in U.S. Pat. No. 4,702,253 a pacemaker which provides a measuring current for measuring the impedance in order to use this impedance measurement to detect the breathing and use it for controlling the pacing rate.
Salo applies, according to U.S. Pat. No. 4,686,987, a similar impedance measuring method to determine the stroke volume of the right ventricle.
Lekholm also uses an impedance measurement for detecting the breathing rate according to U.S. Pat. No. 4,697,591.
In U.S. Pat. No. 4,694,830, the same inventor describes a pacemaker in which the particular impedance can be detected from the change in the stimulation voltage and the stimulation current during each generated stimulation pulse by division of the two stated values, and from the change therein the breathing rate can be indirectly detected, the latter then being used to control the pacing rate. However, this is only successful in cases in which pacing is effected only by the pacemaker. Cardiogenic heart beats cannot be used with this method for rate control and for detecting the breathing rate. This is an essential limitation of this system.
H. Strandberg, et al., in U.S. Pat. No. 4,757,815 controls pacing in accordance with amplitudes of QRS complex signals and varies the rate of the pacing pulses in response to shifting frequency of the respiration signal as acquired from fluctuations in the heart signal derived between a single electrode tip in the heart and a pacemaker housing. Respiration signals are thus subject to false variations such as by swinging the arms. Furthermore there is no fail-safe check of the respiration rate against any other sensed values to determine if it is an optimal control signal for pacing when starting or stopping strenuous exercise for example when the breathing rate is not necessarily an optimal indicator for pacing rate.
Most of the aforesaid systems use the detectable change in impedance to determine the breathing rate and use it for control of the pacing rate of the heart. An atrially synchronous control of the heart rate in the ventricle is not possible with such systems alone, since they have no suitable measures for detecting the atrial activity as well.
The invention is based on the problem of reliably controlling a cardiac pacemaker of the type in question with control signals for the pacing rate obtained from a selected optimum one of different cardiac activities with simple measuring technology and low operative effort and energy resources, and which furthermore provides the possiblity of perceiving the signals of the atrium without an additional atrial electrode and using only one probe to the heart.